This disclosure relates to as-crystallized zeolite compositions of the RHO-type (hereinafter referred to as RHO zeolites), ion-exchanged RHO zeolites made from said as-crystallized RHO zeolites, and methods of making the same. The ion-exchanged RHO zeolites disclosed herein are useful as adsorbents in various applications, such as for kinetically separating oxygen from oxygen-containing streams such as for purifying crude argon, for separating nitrogen from air, for removal of trace N2 from argon, and/or for removal of CO2 from methane. In particular, disclosed herein are as-crystallized RHO(3.1-3.6) zeolite compositions with improved morphology that can be ion-exchanged and used as adsorbent compositions, and methods of making the same.
In the industrial gas production industry, there is a need to efficiently separate oxygen from oxygen-containing streams at ambient or sub-ambient temperatures.
In cryogenic air separation, nitrogen (N2), oxygen (O2), and argon (Ar) are separated based on their boiling points and relative volatilities. A first cryogenic column provides a rough separation of the three main components of air: N2 (78%), O2 (21%), and Ar (1%). A side stream is removed and sent to a second column known as the side arm column or crude argon column. This stream is called “crude” because it exits this side arm column at only about 95% argon. The conventional methods for further purifying this crude argon are limited to: “Deoxo” purification, getter bed technologies, and additional distillation. The Deoxo process reacts controlled amounts of hydrogen with the oxygen in the argon stream to form water, which is more easily removed from the stream. Because the reaction of hydrogen and oxygen generates significant heat, this process can be dangerous if not controlled properly. Getter beds only function at lower oxygen concentrations by reacting oxygen with copper catalyst to form copper oxide. When high purity argon is desired, a third distillation column can be used to further concentrate it. Unfortunately, these distillation columns require upwards of 200 stages due to the similarity in boiling points of O2 and Ar and are less economical than is desired.
Compared to the conventional, very elaborate methods of recovering argon from a crude argon stream, a PSA (pressure swing adsorption) process provides a simple and effective alternative for argon purification and recovery. No hydrogen or additional cryogenic stages are required.
However, to achieve a kinetic or adsorption rate-dependent separation of O2 from either N2 or Ar by an adsorption mechanism, an adsorbent structure must be developed with very specific pore dimensions. The Lennard-Jones 6-12 kinetic diameter of Ar (3.40 Å) is smaller than that of O2 (3.46 Å), but O2 is not a spherical molecule and has a minimum molecular dimension that could be exploited. The symbol A represents the Angstrom, a unit of length, which is defined as 10−10 meters. Adsorption mechanisms suggest that the minimum molecular dimension is the limiting factor for kinetic exclusion. With the proper orientation, O2 should diffuse into a pore with an effective diameter of 2.8 Å. Argon, a spherical atom, will have a constant diameter of 3.4 Å. This 0.6 Å difference in diameters is the key sensitivity that an O2 selective adsorbent must demonstrate to achieve a kinetic separation between oxygen and argon. With such an adsorbent, a process could be derived that purifies crude argon from the cryogenic air separation process in a safer and more economical manner and removes O2 from argon much more rapidly and efficiently.
U.S. Pat. No. 5,730,003 describes a hybrid process where crude argon produced in a cryogenic distillation plant is processed in a 2-bed pressure swing adsorption (PSA) unit to produce 99.999% argon. If the crude argon contains significant amount of nitrogen in addition to oxygen, the patent reports to include a nitrogen selective adsorbent in a layer separate from the oxygen selective layer. Carbon molecular sieve (CMS), type A zeolite, clinoptilolite, and the adsorbents disclosed in U.S. Pat. No. 5,294,418 are used as an oxygen selective layer. As a nitrogen selective layer, adsorbents such as CaA, type X zeolite (LiX or NaX), and zeolite of type A & X containing mixed cations selected from groups I and II of the periodic table (LiNaX) are mentioned.
U.S. patent application Ser. No. 15/718,467, RHO ADSORBENT COMPOSITIONS, METHODS OF MAKING, AND USING THEM, and Ser. No. 15/718,620, PROCESSES USING IMPROVED RHO ADSORBENT COMPOSITIONS (the contents of which are incorporated herein by reference), describe novel RHO zeolite compositions, as well as previously described RHO zeolite compositions, and their application to PSA processes and specifically PSA processes for the removal of oxygen from oxygen-containing fluid streams.
A RHO zeolite has a symmetric, three-dimensional pore structure containing channels with openings made up of two, 8-membered oxygen rings, and in the as-crystallized form contains sodium and cesium cations. The nominal ring diameter or opening is 3.6 Å. This is close to the target pore dimensions, mentioned above, for the kinetic separation of O2 from Ar and N2, and N2 from Ar vide supra. This pore dimension could also be useful in the separation of CO2 from methane.
The as-prepared, hydrated, RHO zeolites crystallize with a centrosymmetric body centered cubic (bcc) structure, but it has been shown that this structure can undergo rather large distortions to lower symmetry upon dehydration and if subjected to certain types of extra-framework cation substitution. The distortion, which can be observed as a large unit cell contraction, is largely driven by the distortion of the RHO 8-rings. Corbin and coworkers have shown that the undistorted, essentially circular rings of the proton-exchanged RHO can distort to highly elliptical rings on exchange of small, high charge density cations such as Ca2+ and Li+ (Journal of the American Chemical Society, 1990, 112, 4821).
RHO zeolites require the presence of large cesium extra-framework cations as the structure directing agent during synthesis, and do not occur naturally. They were first prepared in 1973 by Robson and coworkers (Advances in Chemistry Series, 1973, 121, 106.). This initial synthesis used no additional organic templating agents and produced RHO materials with a ratio of Si to Al atoms equal to 3.1 to 3.2, hereafter specified by the shorthand RHO(3.1) to RHO(3.2). Unfortunately, the template-free method of Robson has proven somewhat unreliable, often producing mixtures of RHO and other cesium-containing phases, such as pollucite, as well as a mixture of crystalline morphologies and particle sizes.
Corbin and coworkers (Journal of the American Chemical Society, 1990, 112, 4821) describe a modified, template-free synthesis of NaCsRHO(3.2), which uses a soluble alumina source, sodium aluminate, and somewhat higher water content. This method more reliably produces pure zeolite RHO than the method of Robson vide supra, but the present inventors have found that it still leads to mixed RHO morphology phases. Moreover, the present inventors have found that these mixed morphology phases persist through subsequent ion exchange processes such as those required to make the ion-exchanged RHO compositions described in U.S. patent application Ser. No. 15/718,467, RHO ADSORBENT COMPOSITIONS, METHODS OF MAKING, AND USING THEM, and Ser. No. 15/718,620, PROCESSES USING IMPROVED RHO ADSORBENT COMPOSITIONS (discussed supra), and that said mixed morphology phases have an adverse effect on the adsorption properties of the ion-exchanged RHO zeolite.
The presence of RHO of mixed morphologies from these existing NaCsRHO(3.2) crystallization routes was explored more fully by Mousavi and coworkers (Mousavi, S. F. et. al, Ceramics International, 2013, 39, 7149). They evaluated the effects of crystallization time, synthesis temperature, water content, and alkalinity during synthesis on the resulting NaCsRHO(3.2) particle shape and size. Though not explicitly cited, the authors follow a synthesis recipe found in Robson (U.S. Pat. No. 7,169,212). It was shown by microscopy studies in this study that RHO(3.2) naturally crystallizes concurrently in two different particle morphologies: 1) polyhedral crystallites of presumably uniform density and 2) polycrystalline aggregates of roughly the same overall size but composed of many small crystallites grown together. X-ray diffraction data presented in the study indicates uniform NaCsRHO(3.2), from which will be understood by those skilled in the art that both particle morphologies are RHO(3.2) with the same chemical composition, within the detection limits of XRD.
The present inventors have found that the presence of impurity phases in and even multiple morphology phases in the, as-crystallized, pure NaCsRHO(3.2), make the attainment of consistent and uniform adsorption properties in subsequent ion-exchanged RHO forms difficult.
More recently, RHO zeolites have been synthesized by Chatelain and coworkers using 18-crown-6 as a templating agent (Microporous Materials, 1995, 4, 231). The templated method more reliably gives highly crystalline NaCsRHO with Si/Al=3.9 to 4.5, i.e., RHO(3.9) to RHO(4.5), and very uniform particle sizes and morphologies, but is challenged commercially by the high cost of the 18-crown-6 templating agent and the added unit operation and difficulty fully removing the templating agent by calcination at high temperature.
Accordingly, there remains a need in the art for as-crystallized RHO(3.1-3.6) zeolites of consistently high purity with uniform particle sizes and morphologies, and for organic-template-free methods for producing the such zeolites, which zeolites can then be ion-exchanged (as for example described in U.S. patent application Ser. Nos. 15/718,467 and 15/718,620, discussed supra) to provide improved adsorbents that are particularly suited for the separation of O2 from mixtures containing N2 and/or Ar.